1. Field of the Invention
The invention relates to the field of digital watermarks, and more particularly, to a Discrete Fourier Transform (DFT) watermark.
2. Description of the Related Art
(This application references a number of publications, as indicated in the specification by a reference number enclosed in brackets, e.g., [x]. These publications, along with their associated reference numbers, are identified in the section below entitled “References.”)
With the recent growth of networked multimedia systems, techniques are needed to prevent (or at least deter) the illegal copying, forgery and distribution of media content comprised of digital audio, images and video. Many approaches are available for protecting such digital data, including encryption, authentication and time stamping.
One way to improve a claim of ownership over digital data, for instance, is to place a low-level signal or structure directly into the digital data. This signal or structure, known as a digital watermark, uniquely identifies the owner and can be easily extracted from the digital data. If the digital data is copied and distributed, the watermark is distributed along with the digital data. This is in contrast to the (easily removed) ownership information fields allowed by the MPEG-2 syntax.
Digital watermarking is an emerging technology. Several digital watermarking methods have been proposed.
For example, Cox et al. in [1] proposed and patented a digital watermark technology that is based on a spread spectrum watermark, wherein the watermark is embedded into a spread spectrum of video signals, such as Fast Fourier Transform (FFT) or Discrete Cosine Transform (DCT) coefficients.
Koch, Rindfrey and Zhao in [2] also proposed two general watermarks using DCT coefficients. However, the resulting DCT has no relationship to that of the image and, consequently, may be likely to cause noticeable artifacts in the image and be sensitive to noise.
A scene-based watermark has been proposed by Swanson, Zhu and Tewfik in [3]. In this method, each of a number of frames of a scene of video data undergoes a temporal wavelet transform, from which blocks are extracted. The blocks undergo perceptual masking in the frequency domain, such that a watermark is embedded therein. Once the watermark block is taken out of the frequency domain, a spatial mask of the original block is weighted to the watermark block, and added to the original block to obtain the watermarked block.
Pereira and Pun [4] proposed inserting a registration pattern in the watermarked image and trying to identify and invert the geometric distortion before watermark detection. As pointed out by Lin et al. [5], there are problems with this solution. One problem is that it may reduce image quality due to insertion of the registration pattern. Another problem is the possibility of a collusion attack, because all the watermarked images share a common registration pattern.
In Lin [5], a Fourier-Mellin transform was used to embed a watermark, which was cast in a rotation-, scale-, and translation-invariant domain. To solve the problem of instability in inverting a log-polar re-sampling of the Fourier magnitude, an iterative method was proposed to approximate this step. However, this approach is computationally complex, which makes it unsuitable for realtime applications.
Earlier, Solachidis and Pitas [6] proposed a circular symmetric watermarking scheme in the DFT domain where each watermark bit was embedded in a section of a circular ring in the frequency domain. In the case of rotation, a search for the rotated angle needed to be performed to detect the watermark.
Regardless of the merits of prior art methods, there is a need for an improved watermark for digital data that prevents copying, forgery and distribution of media content. The present invention satisfies this need. More specifically, the goal of the present invention is to provide unique, dynamic and robust digital watermarks for digital data, in order to trace any compromised copies of the digital data.